OP I'm so sorry you lost your mother at such a pivotal time in your life.
Like other PP's I can totally understand why you fell into this routine and the comfort it must have brought you.
It also sounds, from your posts, that these visits are far from maudlin, rather they have a multiple purpose (walking the dog, going to the park to play and paying your respects to your DM as you pass).
I can understand that you don't want to question your child further so perhaps a suggestion.
Next scheduled visit walk a different way to the park. See what he says/doesn't say.
Does he notice you've gone a different way and not visited the grave?
I think it's hard to advise atm because it's not clear what if it's the regular visits he's now uncomfortable with or specifically the idea of going on Christmas Day.
It may well be the latter (on a normal visit he goes to the park, gets buttons and lovely time with you, but at Christmas I'm guessing he just wants to be cozy at home playing with his new toys).
I think the key thing is to keep an open mind (which it seems you are doing) and perhaps view this as an opportunity for you both to move forward.
I'm not suggesting "move forward" as moving beyond grieving (sadly that doesn't happen imho) but rather in establishing new routines and ways to enjoy time with your son and still privately being able to acknowledge your mother.
It's very personal but I have found great comfort in a fruit tree I planted in the garden and can tend in memory of a loved one.
I make jam with the fruit and name the jam after the person I lost (and give jars to other family members who love it too - not just for the jam but the sentiment). I love it when it blossoms etc and enjoy sitting outside in the summer admiring it (especially if I can have "their jam" an clotted cream on scones!)
Or as others have suggested a lovely candle you can light when the kids are in bed.
Best wishes - you've been through a lot and sound like a lovely Mum x